1. Field of the Invention
The invention relates to a four-way spool type valve for controlling the respective application of fluid pressure and exhaust pressure to opposite sides of a double acting piston disposed within a cylinder, wherein the pilot valve for controlling the movements of the valve spool is contained within an axial bore extending through the valve spool.
2. Description of the Prior Art
A four-way hydraulic valve for the selective application of fluid pressure and exhaust pressure respectively to opposite sides of a double acting piston-cylinder combination has been heretofore utilized in many industrial applications. Where substantial fluid pressures are involved, the four-way valve generally has to be of substantial size in order to withstand the pressures applied to the double acting cylinder. Such control valve has generally taken the form of an outer housing containing axially spaced radial ports which are respectively connectable to the pressure and exhaust side of a fluid pressure source and the opposite sides of the cylinder containing the double acting piston. The flow of fluid through the proper ports is normally controlled by an axially shiftable spool mounted within the ported housing and having external sealing elements for effecting the required separation of the various ports in accordance with the axial position of the spool relative to such ports. To effect the rapid shifting of the valve spool in order to expedite the application of proper control pressures to the double acting cylinder, it has been common to apply differential fluid pressures to the ends of the spool through the medium of a relatively small pilot valve, which is operated either manually or by an electrical or hydraulic actuator. The employment of a separate pilot valve inherently involves the utilization of separate fluid pressure conduits leading from the pilot valve to the main spool type valve, with an attendant increase in total bulk of the valving package, increased cost of assembly, and increased risk of failure of the valve due to rupture of the exposed external piping.
There is a need, therefore, for a four-way spool type hydraulic valve employing a pilot valve to effect the control movements of the spool of the four-way valve, wherein the pilot valve may be incorporated as an internal part of the spool valve and any external conduits connecting the pilot valve to the spool valve may be eliminated.
Moreover, there is often a need for convenient adjustment of the speed of operation of a double acting cylinder controlled by a four-way spool valve.